Much effort has been invested in techniques for accurately tracking and drilling boreholes that are to be positioned relative to existing boreholes. Many such techniques rely on the conductivity or ferromagnetism of steel tubing in the reference borehole, yet the range at which such techniques operate has proven to be undesirably limited. In an effort to extend this range, attention has recently turned to the use of one or more beacons in the casing of a reference borehole. See, for example, U.S. Pat. No. 7,568,532 “Electromagnetically determining the relative location of a drill bit using a solenoid source installed on a steel casing” by Kuckes et al, which discloses the use of ranging for drilling parallel wells for steam-assisted gravity drainage (SAGD), yet this solution is unsuitable for one of the more urgent applications of ranging techniques.
Before a borehole can be cased, it must be drilled. It is during the drilling process itself when well control issues, such as pressure kicks or blowouts, are most likely to be encountered. In many cases, such control issues are so severe as to merit the drilling of a relief borehole to intersect the initial borehole in order to inject a dense “kill” fluid that suppresses a further influx of formation fluid. As a general principle, it is desirable to intersect the target borehole as near to the source of the well control issue as possible, yet during the drilling process this source is most likely located in the open portion of the borehole, long before that portion of the hole can be provided with a casing beacon and a power supply line. Often the drill string remains, but as previously noted, techniques relying solely on the ferromagnetic or conductivity properties of the drill string have an undesirably limited range.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.